【TED-Ed】宇宙尘埃 | The dust bunnies that built our planet

    【TED-Ed】宇宙尘埃 | The dust bunnies that built our planet

    考虑你现在所坐的位置。时光倒流,它可能被淹没在浅海底部,埋在数英里深的岩石下,或者漂浮在熔融的景观中。但是回到46亿年前,你会被淹没在中间环绕着一颗新生恒星的一个巨大尘埃云和气体云中。这个宇宙尘埃到底是什么?洛林·马修斯(Lorin Matthews)为我们探究了一番。

    Consider the spot where you’re sitting. Travel backwards in time and it might’ve been submerged at the bottom of a shallow sea, buried under miles of rock, or floating through a molten, infernal landscape. But go back far enough— about 4.6 billion years, and you’d be in the middle of an enormous cloud of dust and gas orbiting a newborn star. This is the setting for some of the biggest, smallest mysteries of physics: the mysteries of cosmic dust bunnies.


    Seemingly empty regions of space between stars actually contain clouds of gas and dust, usually blown there by supernovas. When a dense cloud reaches a certain threshold called the Jeans mass, it collapses in on itself. The shrinking cloud rotates faster and faster, and heats up, eventually becoming hot enough to burn hydrogen in its core. At this point a star is born. As fusion begins in the new star, it sends out jets of gas that blow off the top and bottom of the cloud, leaving behind an orbiting ring of gas and dust called a protoplanetary disk. This is a surprisingly windy place; eddies of gas carry particles apart, and send them smashing into each other. The dust consists of tiny metal fragments, bits of rock, and, further out, ices.


    We’ve observed thousands of these disks in the sky, at various stages of development as dust clumps together into larger and larger masses.


    Dust grains 100 times smaller than the width of a human hair stick to each other through what’s called the van der Waals force. That’s where a cloud of electrons shifts to one side of a molecule, creating a negative charge on one end, and a positive charge on the other. Opposites attract, but van der Waals can only hold tiny things together.


    And there’s a problem: once dust clusters grow to a certain size, the windy atmosphere of a disk should constantly break them up as they crash into each other. The question of how they continue to grow is the first mystery of dust bunnies.


    One theory looks to electrostatic charge to answer this. Energetic gamma rays, x-rays, and UV photons knock electrons off of gas atoms within the disk, creating positive ions and negative electrons. Electrons run into and stick to dust, making it negatively charged. Now, when the wind pushes clusters together, like repels like and slows them down as they collide. With gentle collisions they won’t fragment, but if the repulsion is too strong, they’ll never grow. One theory suggests that high energy particles can knock more electrons off of some dust clumps, leaving them positively charged. Opposites again attract, and clusters grow rapidly.


    But before long we reach another set of mysteries. We know from evidence found in meteorites that these fluffy dust bunnies eventually get heated, melted and then cooled into solid pellets called chondrules. And we have no idea how or why that happens. Furthermore, once those pellets do form, how do they stick together? The electrostatic forces from before are too weak, and small rocks can’t be held together by gravity either. Gravity increases proportionally to the mass of the objects involved. That’s why you could effortlessly escape an asteroid the size of a small mountain using just the force generated by your legs. So if not gravity, then what? Perhaps it’s dust. A fluffy dust rim collected around the outside of the pellets could act like Velcro. There’s evidence for this in meteors, where we find many chondrules surrounded by a thin rim of very fine material– possibly condensed dust.


    Eventually the chondrule pellets get cemented together inside larger rocks, which at about 1 kilometer across are finally large enough to hold themselves together through gravity. They continue to collide and grow into larger and larger bodies, including the planets we know today.


    Ultimately, the seeds of everything familiar– the size of our planet, its position within the solar system, and its elemental composition– were determined by an uncountably large series of random collisions. Change the dust cloud just a bit, and perhaps the conditions wouldn’t have been right for the formation of life on our planet.


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